**2.6 Acidification of ONS**

The pH indicates the degree of acidity or basicity of the solutions and is relevant by the availability of plant nutrients. **Figure 4** shows pH behavior in sequential extractions, in both composts (bovine and poultry). The pH range was 7–7.8, neutral to alkaline, indicating the possible presence of ions such as Ca2+, Na+ , Mg2+, HCO3<sup>−</sup>, and CO3 <sup>2</sup><sup>−</sup>. The pH suitable for most plants in hydroponic systems is between 5 and 6 (Rodríguez et al., 2006), so organic nutrient solutions must be acidified that will partially eliminate carbonates and bicarbonates. Chemical or organic acids can be utilized; in the case of the test we conducted with bovine and poultry manure, the amount of sulfuric acid that we applied to lower pH from 7.4 to 6 per liter of ONS is 0.1 μL or 60 mL of acetic acid.

#### **2.7 Nutrient supplementation**

With the measurement of EC and pH, hydroponics solutions can be assessed. Up to this point, it is possible to use the nutritive solution obtained in small- and medium-sized plants, such as baby lettuces shown in **Figure 5**.

To produce higher biomass plants, for example, solanacea, cucurbitaceae, etc., it is recommended to have an analysis of the contents of essential elements in order to supplement in the organic nutrient solution, which can be very variable as shown in **Table 3**. The organic nutrient solutions are deficient in most essential elements when compared to known nutrient solutions. Therefore, it is necessary to supplement them with organic or inorganic sources, depending on the production system being worked.

**Figure 4.** *pH of concentrates obtained sequentially from poultry and bovine compost.*

#### **Figure 5.**

*Twenty days after transplanting baby lettuce produced with chemical synthetic fertilizer (A) and manure extract (B). Both cultivated in floating root system with pH 6 and EC = 1.5 dS m<sup>−</sup><sup>1</sup> .*


*(1) Pant [5]; (2) Pant (2011); (3) González et al. [6]; (4) Ochoa et al. [7]. OM = organic material; VCG = vermicompost poultry manure; VCG = vermicompost poultry manure; CBv = compost bovine*

#### **Table 3.**

*Macro- and micronutrient content in some organic nutrient solutions.*
